1. Field of the Invention
The present invention relates to thermoformed cellular polyurethane. More particularly, the invention relates to the formation of shaped polyurethane parts, especially panels, by the thermoforming compression of polyester-urethane foams. The resulting shapes and panels are advantageously used in the railway, automotive, and aircraft industry as headliners, trim panels, engine compartment covers, and the like.
2. Description of the Prior Art
Formed sheets such as trim panels, headliners, and vehicle trim have exhibited great utility in the automotive, aircraft, and railway industries. As described in German Pat. No. 2,602,839 (U.S. Pat. No. 4,059,660 and U.S. Pat. No. 4,119,749), such sheets may be formed by laminating smooth sided corrugated paper with a foam layer, for example, a layer of polyurethane foam. An adhesive is utilized to adhere the various layers following which the components are bonded together under pressure at room temperature and formed to the desired shape. The preparation of these sheets is highly labor-intensive and is unsuitable for large-scale production.
Reversibly thermoformable fiber-reinforced rigid polyurethane plastics may be prepared according to German Pat. No. 2,164,381 (Great Britain Pat. No. 1,411,958) by incorporating inorganic or organic fibers in a partially reacted polyurethane system which is liquid up to 50.degree. C., containing primarily bifunctional polyols having hydroxyl numbers from 100 to 600, and modified polyisocyanates or polymeric diphenylmethane-diisocyanates. The disadvantage of this two-step process is that the fiber reinforcing material, whether woven or non-woven, must first be coated with a flowable polyurethane reaction mixture followed by curing in a closed mold at temperatures over 120.degree. C. The flat sheets produced in this manner can then be thermoformed at temperatures from 130.degree. C. to 220.degree. C. However, this cost-intensive process also limits the rate at which molded parts may be produced to a low number of parts per unit time.
German Pat. No. 2,607,380 (U.S. Pat. No. 4,129,697) describes the preparation of thermoformable polyisocyanurate foams by reacting polyether polyols, glycols, and diphenylmethane diisocyanate, which can contain up to 20 weight percent closely related polyisocyanates of higher molecular weight. Expansion takes place in heated molds or on conveyors, whereby the foams are post cured or tempered for approximately 15 minutes at 80.degree. C. One of the disadvantages of this process is that the polyisocyanurate foams are brittle, do not exhibit internal cushioning, and are poor sound absorbers. Moreover, polyisocyanurate slab foams cannot generally be produced with slab thicknesses greater than 50 cm since otherwise foam core discoloration can occur.
It is difficult or even impossible to use in-mold foaming to produce flat structures with wall thicknesses from 1 mm to 6 mm suitable for trim panels, headliners, engine compartment covers, since this would lead to very high gross densities. In addition, it is extremely difficult to completely and uniformly fill molds having complicated shapes with relatively high-viscosity polyurethane mixtures. If this were possible at all, very high pressure would be required.
The object of this invention is to prepare cellular, self-supporting, polyurethane shaped objects, having large surface area and low densities, whereby said objects may be produced economically at high volume. The polyurethane shaped objects should possess a high degree of sound absorption, good thermal insulation properties, high flame resistance and the ability to recover from compression loads. Polyurethane foams suitable for use as the initial components ought not to exhibit core discoloration or scorching.